Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations only say readOnlyHint=false, destructiveHint=false and openWorldHint=true; the description goes well beyond that by disclosing a real cost ($3/month from credits), the side effect of provisioning a number, and a consent requirement before the first purchase. It also implies idempotent get-or-create behavior, so a repeat call is not a surprise charge.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

Three short sentences with no filler, and the most decision-relevant fact (cost plus confirm-before-buying) is placed after the identity sentence rather than buried. The elliptical phrasing "or their mobile's" is slightly clipped but readable in context.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a single optional parameter with no output schema, this covers cost, side effects, defaults, and consent — the things an agent needs to act safely. The one gap is that it never says what comes back on success (the number itself, or a confirmation), which matters since there is no output schema to fall back on.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100% and already documents area_code as optional 3 digits, so the baseline is 3. The description adds genuine meaning: the fallback is the person's mobile area code, and the value is a preference applied at provisioning time rather than a filter on a lookup.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

The description names the resource precisely ("Your person's own phone number, answered by their agent") and clarifies the get-or-provision behavior in the second sentence, which separates it from siblings like my_mobile and who_can_reach_me. The verb is implied rather than stated up front, so an agent has to read two sentences to learn this both retrieves and creates.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

It gives a concrete precondition — confirm with the person before the first purchase — and explains the conditional branch (gets one only if they don't already have one). It never explicitly names an alternative tool or says when NOT to call it, leaving the boundary with my_mobile to inference.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.